feat(notifications): webhook custom headers

[0xSysR3ll]

Description

This PR adds the ability to configure custom HTTP headers for webhook notifications.

• Supersedes feat(webhook): add API key support for webhook notifications #1948

How Has This Been Tested?

• Added custom headers via the UI and verified they appear in webhook requests
• Tested adding/removing multiple headers
• Verified empty headers are automatically filtered out on save
• Confirmed Authorization header (authHeader ) takes precedence over custom Authorization headers
• Tested webhook test notification with custom headers (used a simple python webhook listener script to inspect headers sent in requests)

Screenshots / Logs (if applicable)

Checklist:

• I have read and followed the contribution guidelines.
• Disclosed any use of AI (see our policy)
• I have updated the documentation accordingly.
• All new and existing tests passed.
• Successful build pnpm build
• Translation keys pnpm i18n:extract
• Database migration (if required)

[M0NsTeRRR]

Confirmed Authorization header (authHeader ) takes precedence over custom Authorization headers

In my opinion, it would be better to prevent saving if the system detects that authentication headers are already configured.

[0xSysR3ll]

Confirmed Authorization header (authHeader ) takes precedence over custom Authorization headers

In my opinion, it would be better to prevent saving if the system detects that authentication headers are already configured.

Mmm not a bad idea ! I'll add this :)